they're replacements. if you've seen those on any strat it's because of two things, either the owner wanted more mass to increase sustain or they're graph techs to decrease the incidents of string breakage.

Thats a 2000 american strat but if you look closer at the pic of the bridge you can see the six screws that hold down the original Fender bridge. Fender does sell an aftermarket bridge with those type saddles but it has a pivot point on each end. Someone replaced those saddles.